Belts were different by a few inches over the years depending on pulley sizes, mounting brackets, etc. My recommendation is to remove the belt (including AC belt as recommended above). Stretch the belt flat (doubled) & measure, multiply by 2. Go to a website like Ryder Fleet Products
& find their wide ribbed belts selection. Click on a few examples to ascertain the "nomenclature" of the part number/name. E.g. the Dayco 5080780
(second belt in second row from link above) is an 8 rib x 78" belt. If you need a 76" belt, you'd be looking for a 5080760, a 76.5" would be 5080765, etc. You'll also notice at the bottom of the belt description is a list of "interchange" part#'s; some of those look suspiciously like Cummins part#'s and some like competing mfgr's parts.
Take the v-belt to an autoparts store, they should be able to match it over the counter. If they can't, find a better auto parts store (I find NAPA has the best training at least at stores I've frequented).